In order to improve pressure-resistant characteristics and to prevent electric discharges caused by a foreign material produced by welding sparks in fixing an inner magnetic shield to a frame (a shadow mask frame) of a cathode ray tube by welding, a method for fixing an inner magnetic shield to a frame using spring fixing members (hereafter referred to as fixing members) has been proposed as disclosed, for example, in Japanese laid-open patent application (Tokkai Hei) 5-151908.
That is, the frame and the inner magnetic shield have a first and a second flange portion projecting in the direction crossing the tube axis of the cathode ray tube at right angles, respectively. About 10 engagement holes whose diameter is 10 mm or less are provided on each side of the first and second flange portions. Generally .OMEGA.-shaped fixing members are firmly fit into the engagement holes, thereby fixing the inner magnetic shield to the frame by utilizing spring elasticity. According to this method, since the inner magnetic shield can be fixed without welding, electric discharges caused by a foreign material produced by welding sparks can be prevented.
Conventionally, in consideration of mass-productivity, etc., an inner magnetic shield has been manufactured by punching a sheet by a press. In the case of the press working, burrs are produced on an end face of a part being punched in punching a sheet. Generally, the material for a fixing member is harder than that for an inner magnetic shield. Therefore, burrs on an end face of an engagement hole on an inner magnetic shield drop off when a fixing member comes into contact with the end face of the engagement hole in fitting the fixing member into the engagement hole in order to fix an inner magnetic shield to a frame.
When burrs that are metal scraps drop off, the burrs are caught in the openings of a shadow mask and the openings may be closed. Even if the burrs are not directly caught in the openings of the shadow mask, the burrs may intrude into a slight space between the shadow mask and the frame. The burrs may be come out from the space between the shadow mask and the frame by vibration, impact or the like in the subsequent steps such as, for example, a frit seal process wherein a panel and a funnel are glued, a sealing process wherein an electron gun is installed, an evacuation process wherein a valve is evacuated, and the like. Then, the burrs may be caught in the openings of the shadow mask. When the burrs are caught in the openings of the shadow mask, which causes the openings of the shadow mask to be closed, an electron beam is shielded and the fluorescent substance of the screen does not emit light. That is to say, it causes a picture defect.
In collecting and analyzing the foreign materials that had closed the openings of the shadow mask, about 30% of the foreign materials were burrs that were metal foreign materials. In order to remove burrs, for example, a blasting treatment, chemical polishing, electrolytic polishing and the like have been carried out, since it is not possible to prevent the production of burrs in the press working. In the case of the blasting treatment, there is a problem that an inner magnetic shield is deformed by putting the polishing material on the inner magnetic shield at high pressure. In case of chemical polishing, electrolytic polishing or the like, the processes such as washing, neutralization, washing, etc. are needed after the polishing treatment. If these processes after the polishing are not carried out properly, in the blackening film treatment, which is a surface treatment of the inner magnetic shield carried out in a subsequent process, a nonuniform film in quality may be obtained, the film may drop off or the like. Moreover, many processes for these treatments are needed, which causes cost increases.